Replace a drill chuck by removing the retaining screw, inserting an a 3/8-inch Allen wrench into the chuck and tightening it, and then hitting the wrench with a mallet to loosen the unit from the threads. Unscrew the old chuck from the drill, and replace it with the new one. Insert the retaining screw, and tighten it in place so the drill is ready for use again.
Access the retaining screw by opening the chuck fully. Shine a flashlight through the jaws to determine the type of screw head, and use the appropriate screwdriver to remove it from the drill. If there is no screw, the drill has a tapered spindle, and the chuck cannot be removed using this process. These screws use reverse threads, so hold the chuck, and turn the screwdriver or Allen wrench clockwise to loosen and remove it.
The chuck screws onto a spindle with most portable drills, and the rapid movement of hitting the Allen wrench with a mallet loosens it. Set the drill to low speed, and tighten the clutch to the highest setting before performing this procedure. For a broken chuck that cannot tighten on the wrench, place locking pliers around the chuck, and hit the pliers with the mallet.